Currently, a process known as the four-color separation process is very widely used in the printed reproduction of colored pictures, transparencies and the like. The four-color separation process is generally responsible for all of the high-quality colored reproductions in magazines and books, and is also used for some newspaper work as well.
In the most common version of this process, the original print or transparency is photographed through different filters to produce different individual films which correspond to the basic colors of the four-color separation process: yellow, magenta, cyan and black. The filters utilized to extract the first three of these colors from the original have tints which are the complementary colors of the colors being drawn out. Thus, a green filter is used to pick out the magenta, a blue filter is used to pick out the yellow, and a red filter is used to pick out the cyan. A combination of all filters is ideally utilized to pick out the black, although in some processes the black film is made by photographing the original in black-and-white film, without any filter.
Some "fine tuning" or adjustment of the intensities of the various colors in the process is made by selecting exposure times and development times. Also, in some instances the final inks used to print the final reproduction can be varied and selected to attain certain effects.
From the four pieces of film produced through this process, printing plates are made, these being subsequently attached to plate cylinders in a typical printing machine, which is then able to print the reproduction using the process colors; yellow, magenta, cyan and black.
Generally speaking, the printing industry is of the view that proper reproduction of any photograph or the like requires a four-color separation process of the kind just described, utilizing the process ink colors.
However, in certain branches of the printing industry, particularly in newspapers, shopping bags, the Yellow Pages and advertising flyers, the full four-color separation process represents a considerable expense, since it requires the material to be passed through four printing stations, in order to receive the four colors. This in turn requires the production of four plates, and the time required to mount them, adjust the components, etc. All of this represents a substantial cost factor which, for obvious reasons, it would be of advantage to reduce.
There is a two-ink process forming part of the prior art, known as duotone. In one version of this process, often called "Fake" duotone, a black and white original (for example a photograph) is first photographed on "Ortho" film through a contact screen to give a screen film. "Ortho" film is a high contrast film which is not sensitive to the red region of the spectrum. Then, the same screen is rotated through 30.degree. and another screen film is taken, substantially identical to the first, also on "Ortho" film. The two films are developed to different densities, then are used to make plates which are run in two colors, for example, red and black.
In another version of duotone, often called "Real" duotone, the original art is already in two colors, for example, red and black. The first step is to shoot "Ortho" film through a grey contact screen without any filter. Both the red and the black will be seen by the film, and the result will be a film in which the red and the black are both picked up as black. Then, a panchromatic film is exposed through a red filter and a grey contact screen. In this arrangement, the film sees only black.
The two films are then used to make plates which print red and black, respectively.
It is also known, particularly in the food advertising area where blue colors are rare, to do a three-color separation using the standard filters to obtain yellow, magenta and cyan, and then to print the image using yellow ink, red ink and black ink. In other words, the plate made from the cyan film prints in black ink.
It is further known to mix various colors to produce various other colors. For example, it is known to print a dot matrix of black superimposed on a dot matrix of yellow in order to produce various shades of green. It is also known that red and yellow will combine in the same way to produce orange. It is further known that yellow and cyan will combine to produce various shades of green.
I have now discovered and developed a process by which to attain strikingly realistic reproductions of an original utilizing only two printing plates inked with only two different colors. However, the process of making these plates does not involve simply one of the known parts of the standard four-color separation, nor does it utilize process inks.
By way of explaining this, it should be understood that, if a four-color separation were made to produce four plates intended to print yellow, magenta, cyan and black, and then if only the yellow and red were printed, or only the yellow, blue and black, or any other combination which was not the full combination of four colors, the resulting print would be clearly and definitely unbalanced, and anyone viewing the print would immediately see the unbalanced nature of the colors. The print would appear "too reddish" or "too far into the blue region", or blotchy. The aim of the process which I have developed is to remove the unbalanced nature of a printed reproduction made with only two impressions, and thus two inks.